Getting back to a less powerful Order...
I think that there are plenty of ways to make a less powerful magic system within the rules, things that don't seem to be applied consistently (or are contradictory or controversial or...).
Large spells become ritual seems to be an easy method for doing this. This is either SG fiat or troupe consensus. Direwofl's example almost certainly fits here.
Ritual spells always have a risk of botching. My views on this are well established, but the language in the rule book is contradictory, at best. Some people believe that if you master a spell you can avoid all risk of botch. I don't believe that's the case, but if you want a less powerful magic system, making rituals more risky may certainly make the Order less powerful. Magi then have to invest a fair bit into mastery abilities to negate the risk of a botch. Is your magus really going to risk 10 botch dice on the 10th magnitude ritual spell? Yes, this is problematic for the Aegis of the Hearth, and this is why I'd make Aegis mastery texts readily available to the covenant. There is a reason why the covenant knows a 50th level Aegis and doesn't cast it, unless necessary and relies on the 25th or 30th level Aegis...
Then outside of the rules...
Adjusting the aura system as I described here. While it may seem to be harsh, keep in mind the goal is to reduce the power of the Order and magi, magi will still be powerful, but significantly less so in a foreign aura, or when affecting a foreign aura. I like the idea of magi being holed up in their places of power because they are secure in their magic there, but the "real" world is much more unpredictable for them. The typical city has a divine aura of 3, which when translted into an Aura bonus as finally suggested by The Fixer becomes a 6, and multiplies out to a -18.